In the hydrometallurgical extraction of zinc from zinc-containing metallurgical ore, usually zinc blende, is roasted to produce a calcine which is leached with acid, the solution being purified and used as an electrolyte for the electolytic deposition or electrowinning of the zinc from the solution. The zinc, which is deposited on the cathode, is removed and remelted.
In the past the electrowinning step has generally been carried out with electrolytes which, for the most part, consist of sulfuric acid solutions in which the zinc is dissolved, the anodes of the electrolytic cells being composed of lead, the cathodes of aluminum.
Lead anodes are composed of anode materials which generally have been ternary lead alloys containing 0.5 to 1.0% by weight silver and a small amount of a third component in addition to lead which makes up the balance of the alloy.
In the Journal of Applied Chemistry of the USSR, Vol. 25 (1951) p. 429 ff., investigations of certain elements as the third component have been described. These elements are thallium, tellurium, selenium, bismuth, calcium, gold, mercury, strontium, barium, arsenic, tin and cobalt. In volume 25 (1953) p. 847 ff. of the Journal, the use of magnesium and silicon as the third component has been discussed.
Notwithstanding the considerable research which has been undertaken in the past to develop a satisfactory alloy for use as the anode material in the electrowinning of zinc, experience has shown that earlier anode materials suffer significant weight loss during the electrolysis. This weight loss is an indication of the rapid consumption of the anode material and contributes to the formation of slime in substantial quantities of the zinc deposited at the cathode.
Furthermore, the earlier anode materials suffer from the fact that they lack requisite mechanical strength or, where they originally possess sufficient strength, tend to lose it during use. This diminution of the strength of the anode can result in warping and, where the anode and cathode are closely juxtaposed, to short-circuiting and arcing. Even where short-circuiting and arcing do not occur, there is the danger that the current distribution across the cathode may be affected by the warping, thereby causing nonuniform deposition.
From Blei und Bleilegierungen, Springer-Verlag 1962, p. 285 ff., W. Hofmann, one must draw the conclusion that the use of alloying components in lead anodes gives rise to inconsistent phenomena or effects which cannot be predicted. Also, the alloying ingredients are extremely expensive and their use may be contraindicated simply because of the cost factor. In this regard, it should be borne in mind that an average plant may utilize lead alloys for anode materials in amounts of a thousand metric tons or more in the electrodeposition tank shed.